JavaScript
try, catch, and finally

What is try-catch-finally in JavaScript?

In JavaScript, try, catch, and finally are used for handling runtime errors in a graceful way. They prevent your code from crashing by catching exceptions and letting you decide how to respond.

Syntax:

try {
  // Code that might throw an error
} catch (error) {
  // Code to handle the error
} finally {
  // Code that runs no matter what
}

Example 1: Basic try-catch

This example demonstrates what happens when we try to call an undefined function.

try {
  someUndefinedFunction();
  console.log("This line won't be executed.");
} catch (err) {
  console.log("An error occurred:", err.message);
}

An error occurred: someUndefinedFunction is not defined

Why does this happen?

Because someUndefinedFunction doesn't exist, JavaScript throws a ReferenceError. The catch block catches this error and allows us to log or handle it instead of letting the program crash.

Example 2: Using finally

Let's extend our previous example with a finally block to ensure some code runs regardless of an error.

try {
  console.log("Trying to execute risky code...");
  someUndefinedFunction();
} catch (error) {
  console.log("Caught an error:", error.message);
} finally {
  console.log("Finally block always runs.");
}

Trying to execute risky code...
Caught an error: someUndefinedFunction is not defined
Finally block always runs.

Question:

When does the finally block execute?

Answer: The finally block always runs—whether or not an error was thrown or caught. It is typically used for cleanup operations like closing files or releasing resources.

Example 3: No error in try block

What happens if there is no error in the try block?

try {
  console.log("No errors here!");
} catch (err) {
  console.log("Error caught:", err.message);
} finally {
  console.log("This still runs!");
}

No errors here!
This still runs!

Example 4: Re-throwing an error after catch

Sometimes you may want to catch an error, do something, and then re-throw it for higher-level handlers.

function processUserData() {
  try {
    JSON.parse("This is not JSON");
  } catch (err) {
    console.log("Logging error locally:", err.message);
    throw err; // Re-throwing error
  }
}

try {
  processUserData();
} catch (e) {
  console.log("Caught again at higher level:", e.message);
}

Logging error locally: Unexpected token T in JSON at position 0
Caught again at higher level: Unexpected token T in JSON at position 0

Points to Remember

• try is used to wrap code that may throw errors.
• catch handles the error and prevents the program from crashing.
• finally always executes—used for cleanup logic.
• You can nest try-catch-finally blocks and re-throw errors if needed.

Common Use Cases

• Parsing user input (like JSON)
• Working with external APIs that might fail
• Cleanup tasks (closing database connections, timers, etc.)

Quick Quiz

Q: Will finally execute if there is a return statement in try?

A: Yes. finally always executes, even if try or catch returns a value or throws an error.

function demo() {
  try {
    return "Returning from try";
  } catch (e) {
    return "Returning from catch";
  } finally {
    console.log("Finally runs!");
  }
}

console.log(demo());

Finally runs!
Returning from try